Deadline is a word that I acquired and understood to mean the point at which a piece of work must be handed in or completed. If I thought about it, it would seem logical that it came from some military usage to mark the effective range of weaponry in order to push back the enemy hoards. I was therefore surprised to see it used in John Ford's The Sun Shines Bright as an arbitrary line drawn in front of a gaol to stop a lynch mob.

So I looked it up. Only to find that its first usage was as a line around a military prison beyond which a prisoner is liable to be shot.

Thanks for adding this word and its origin to our collection. I really like it. It’s a word so commonly used, yet I had never given a thought as to where it came from and nor had it ever been discussed in all the many years of Wordwizard’s existence.
I’ll give it my official treatment here for the record:

Merriam-Webster Unabridged Dictionary

deadlinenoun

1) A line drawn within or around a prison that a prisoner passes only at the risk of being instantly shot.

2) A guide-line marked on the bed of a printing-press to indicate the limit to which the printing surface may extend.

3) A fixed time limit : a date or time before which something must be done and after which the opportunity passes or a penalty follows. <the deadline for filing income tax returns>

specifically : The time limit after which copy is not accepted for use in a particular issue of a publication. <3 a.m. was the deadline for the newspaper's morning edition. >

4) A group of military vehicles put aside for repair or periodic maintenance.

Origin: (dead + line) originally U.S., first known use 1864 (sense 1)
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The following quotes are from the Oxford English Dictionary and other sources:

<1864 “The ‘dead line’, beyond which the prisoners are not allowed to pass.”—in Congressional Record, 12 January (1876), page 384/1>

<1868 “ Seventeen feet from the inner stockade was the ‘dead-line’, over which no man could pass and live.”—History Civil War U.S. III by B.J. Lossing, page 600>

<1888 “Should he some day escape alive across the dead-line of Winchesters, he will be hunted with bloodhounds.”—The Contemporary Review, March, page 449>

<1889 “The instant he sought to cross the social dead-line.”— The Plantation Negro as a Freeman. Observations on his character, condition and prospects in Virginia by P. A. Bruce, page 45>

<2019 “The relationship is likely to become more significant as a deadline nears for India to comply with U.S. sanctions against Iran, one of India’s main oil providers.”—The Seattle Times (Seattle, Washington), 20 February>